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Two things make communications networks go: electrons and infrastructure. Electrons supply the power, pushing words and data between the sender and the receiver. Infrastructure - wires, towers, antennas, circuits, and processors - provide the conduit for that energy to move as data. If either one of those two critical ingredients encounters, say, a massive hurricane along the Gulf of Mexico, then all sorts of critical technology goes kablooey. Here's the inside story of how one hurricane wreaked telecommunications havoc.

HURRICANE KATRINA: WHAT STAYED ONLINE, WHAT DIDN'T - AND WHY

Emergency Services DispatchIn Harrison County, Mississippi, the radio towers dedicated to first-responder dispatch - police, fire, and so on - were built to handle 5,000 radios in sustained winds of 200 mph. (Katrina maxed out at 175 mph.) Despite problems in isolated areas, most emergency personnel could talk to their HQs. Yet agencies across the region had trouble talking to one another. As the storm moved in, veterans like Pat Sullivan, fire chief in Gulfport, Mississippi, were prepared for a total system failure. "You need to be able to go back and do it old-school," he says. "We had already decided that we would have runners going from fire station to fire station if the communications system went down."

Cellular PhonesCell phone services suffered the most in the storm. Towers fell, antennas and equipment broke away from their foundations, and generators intended as a fail-safe for the loss of the grid were submerged in the hurricane's record 20- to 40-foot storm surge. (They'd been placed on the ground - or on piers 10 feet high.) Though mobile towers called Cell on Wheels are supposed to keep the network online when the fixed versions go down, they didn't get deployed effectively. However, text messaging worked pretty well. Texts use very little bandwidth, and if they encounter a busy signal they queue up instead of dying off.

Radio and TVRadio and local TV stations mainly stayed on the air, even though towers toppled along the coast. In Biloxi, Mississippi, only a few miles east of Katrina's eye, WLOX-TV lost a low-power antenna and its main building was nearly torn in half. But five days after the storm, the station had missed broadcasting only two hours. And even then, WLOX still wasn't off the air - it put a satellite truck outside the studio and beamed to a friendly station 25 miles away for rebroadcast. But who was watching? People with battery-powered TVs and radios - in fact, eight or nine radio stations actually transmitted WLOX's audio feed for a time.

InternetThe major Internet hub in New Orleans sustained surprisingly few wounds, according to Bill Smith, chief tech­nology officer for BellSouth. But there were serious problems at the edges of the network, where users connect. Head-ends - outdoor cable boxes that route DSL lines to homes or neighborhoods - got submerged. Lines got knocked down. DSL circuitry in some offices got shut off (to avoid shorting out) and may stay that way, because dirty connections can short even when they're dry. And where the power was out, DSL was out of luck - BellSouth hadn't installed battery backups because the systems suck so much power.